As used herein, the term asphalt also comprises macadam and tarmac. Asphalt paved road surfaces typically comprise a mixture of asphalt cement (typically a black, sticky, petrochemical binder) and an aggregate comprising appropriately sized stones and/or gravel. The asphalt concrete mixture is usually laid, compressed and smoothed to provide an asphalt paved road surface.
Over time, an asphalt paved road surface can deteriorate as a result of a number of factors. For example, seasonal temperature fluctuations can cause the road surface to become brittle and/or cracked. Erosion or compaction of the road bed beneath the road surface may also result in cracking. Moreover, certain of the chemical constituents incorporated in fresh asphalt are gradually lost over time or their properties changed with time, further contributing to brittleness and/or cracking of the road surface. Where concentrated cracking occurs, pieces of pavement may become dislodged. This dislodgement can create traffic hazards, and accelerates the deterioration of adjacent pavement and highway substructure. Even if cracking and the loss of pavement pieces do not occur, the passage of traffic can polish the upper highway surface, and such a surface can be slippery and dangerous. In addition, traffic-caused wear can groove, trough, rut and crack a highway surface. Under wet highway conditions, water can collect in these imperfections and set up dangerous vehicle hydro-planing phenomena. Collected water also contributes to the further deterioration of the pavement.
Prior to about the 1970's, available methods for repairing old asphalt-paved road surfaces included: spot treatments such as patching or sealing, paving with new materials over top of the original surface, and removal of some of the original surface and replacement with new materials. Each of these methods had inherent drawbacks and limitations.
Since about the early 1970's, with increasing raw material, oil and energy costs, there has been a growing interest in trying to recycle the original asphalt. The world's highways have come to be recognized as a very significant renewable resource.
Early recycling techniques involved removing some of the original surface and transporting it to a centralized, stationary recycling plant where it would be mixed with new asphalt and/or rejuvenating chemicals. The rejuvenated paving material would then be trucked back to the work site and laid. These techniques had obvious limitations in terms of delay, transportation costs and the like.
Subsequently, technology was developed to recycle the old asphalt at the worksite in the field. Some such processes involved heating and are frequently referred to as "hot-in-place recycling" (hereinafter referred to as HIPR).
This technology comprises many known processes and machines in the prior art for recycling asphalt paved surfaces where the asphalt has broken down. Generally, these processes and machines operate on the premise of (i) heating the paved surface (typically by using large banks of heaters) to facilitate softening or plasticization of an exposed layer of the asphalt; (ii) mechanically breaking up (typically using devices such as rotating, toothed grinders; screw auger/mills; and rake-like scarifiers) the heated surface; (iii) applying fresh asphalt or asphalt rejuvenant to the heated, broken asphalt; (iv) distributing the mixture from (iii) over the road surface; and (v) compacting or pressing the distributed mixture to provide a recycled asphalt paved surface. In some cases, the heated, broken material can be removed altogether from the road surface, treated off the road surface and then returned to the surface and pressed into finished position. Much of the prior art relates to variations of some kind on this premise.
Over time, HIPR has had to address certain problems, some of which still exist today. For example, asphalt concrete (especially the asphalt cement within it) is susceptible to damage from heat. Thus, the road surface has to be heated to the point where it was sufficiently softened for practical rupturing, but not to the point of harming it, Furthermore, it was recognized that asphalt concrete is increasingly hard to heat as the depth of the layer being heated increases.
Many patents have attempted to address these problems. See, for example, the following patents, each of which is incorporated herein by reference:
U.S. Pat. No. 3,361,042 (Cutler) U.S. Pat. No. 3,970,404 (Benedetti) PA1 U.S. Pat. No. 3,843,274 (Gutman et al.) U.S. Pat. No. 3,989,401 (Moench) PA1 U.S. Pat. No. 4,011,023 (Cutler) U.S. Pat. No. 4,124,325 (Cutler) PA1 U.S. Pat. No. 4,129,398 (Schoelkopf) U.S. Pat. No. 4,335,975 (Schoelkopf) PA1 U.S. Pat. No. 4,226,552 (Moench) U.S. Pat. No. 4,534,674 (Cutler) PA1 U.S. Pat. No. 4,545,700 (Yates) U.S. Pat. No. 4,711,600 (Yates) PA1 U.S. Pat. No. 4,784,518 (Cutler) U.S. Pat. No. 4,793,730 (Butch) PA1 U.S. Pat. No. 4,850,740 (Wiley) U.S. Pat. No. 4,929,120 (Wiley et al.) PA1 igniting in a burner a combustible mixture comprised of a fuel and oxygen to produce a hot gas; PA1 feeding the hot gas to an enclosure having a radiative face disposed above the asphalt surface, the radiative face having a plurality of apertures; and PA1 selecting the dimension of the apertures such that the hot gas: (i) heats the radiative face to provide radiation heat transfer to the asphalt surface; and (ii) passes through the apertures to provide convection heat transfer to the asphalt surface. PA1 =the convection heat-transfer coefficient; PA1 A=the total surface area of the heater; PA1 T.sub.1 =the temperature of the hot gas; and PA1 T.sub.2 =the temperature of the asphalt surface. PA1 .epsilon.=the total emissivity of the radiative surface; PA1 .sigma.=the proportionality (Stefan-Boltzmann) constant; PA1 A=the total surface area of the heater; PA1 T.sub.1 =the temperature of the radiative face of the enclosure; and PA1 T.sub.2 =the temperature of the asphalt surface.
Regardless of the specific technique used, commercially successful asphalt surface recycling is largely dependent on the ability to heat the old asphalt surface to be recycled in an efficient manner. Generally, efficient heating is achieved when the asphalt surface is heated to the desired temperature (eg. 300.degree. F.) both quickly and without substantial scorching or overheating.
It is conventional in the art to utilize a heater to soften the asphalt thereby facilitating recycling thereof. The heater may be a radiant heater (e.g. infrared heater), a hot air heater, a convection heater, a microwave heater, a direct flame heater and the like.
By far the most popular commercially utilized heater is a radiant heater emitting infrared radiation. Generally, such a heater operates by igniting a fuel/air mixture over a metal (or other suitable material) screen resulting in combustion of the mixture. The heat of combustion is absorbed by the metal screen which, in most cases, results the metal screen glowing red and radiating the asphalt surface with heat (i.e. infrared radiation). One of the significant limitations of conventional radiant heaters is the source of fuel. Specifically, since the fuel/air mixture must be combusted of the entire radiative surface of the heater, the fuel must be of a nature which enables it to be readily mixed with air and distributed substantially evenly over the radiative surface up to the point of ignition. The result of this is that virtually all commercially available radiation heaters are fuelled by propane or butane. Propane and butane are gases which may be readily mixed with air for use in this application.
Unfortunately, propane and butane are very hazardous materials to handle and use since they are typically stored under pressure which can lead to a dangerous explosion in the event of an accidental spark. Further, there are a number of countries in the world in which propane and/or butane are: (i) unavailable, (ii) prohibitively expensive, and/or (iii) unattractive in the face of other available lower cost liquid fuels such as diesel fuel. Indeed, one or more of these problems exist in most countries in the world outside North America, Europe and Australia. With regard to (iii), liquid fuels (i.e. fuels which are liquid at ambient temperature and pressure) are unsuitable for use in conventional radiation heaters due to the difficulties associated with atomizing such fuels in air and distributing the fuel/mixture substantially evenly over the radiative surface of the heater. The net result of this is that HIPR is commercially impractical in most countries in the world outside North America and Europe.
Further, with conventional radiation heaters, the temperature of the radiative surface can easily reach 2000.degree. F. or more. This results from the need to heat the surface as quickly as possible so that the progression of all vehicles associated with the recycling system is not delayed. This, coupled with the need to heat the surface of the asphalt to a temperature of 300.degree. to 400.degree. F. with the ultimate goal of attaining an average temperature of about 250.degree. F. a depth of at least 2 inches, can often lead to scorching or overheating of the asphalt surface. Unfortunately, attempts to obviate this effect simply by lowering the temperature of the radiative surface, leads to even poorer efficiencies in the overall recycling process and thus, is not consideration a commercially viable alternative. A further problem associated with conventional radiation heaters is the high potential for non-uniform heating. Typically, this results from certain areas in the asphalt surface attracting radiation (e.g. oil spots) and other areas reflecting radiation (e.g. light coloured aggregate). The problem is exasperated in areas of the asphalt surface attracting radiation since this typically leads to severe smoking and/or ignition of the asphalt surface thereby creating a significant environmental concern.
As alluded to above, a conventional asphalt surface heater is a hot air heater. Such a heater is described in U.S. Pat. No. 4,561,800 Hatakenaka et al. (Hatakenaka)!, the contents of which are hereby incorporated by reference. Hatakenaka teaches a method of and an apparatus for heating a road surface, in which hot air controlled to a predetermined temperature is blown against the road surface so as to heat the road surface. The apparatus includes a hot air generator provided with a burner and a thermal control unit, and a number of ducts formed with blowing pores for blowing the hot air against the road surface. Hatakenaka purports that the apparatus facilitates reducing the amount of smoke produced during heating of the asphalt surface. A principal consideration in Hatakenaka is the ability to control the temperature of the hot air. Thus, the essence of Hatakenaka is the provision of hot air at a controlled temperature which hot air is used as the means by which the road surface is heated. Hatakenaka asserts that one of the advantages of the invention is the ability to adjust the "thermal capability" of the heater simply by adjusting the temperature of the hot air itself. This underlies the notation that, for all intents and purpose, Hatakenaka relates to an apparatus which provides substantially all heat by convection.
One of the principal difficulties with hot air and convection heaters generally, and the apparatus taught by Hatakenaka specifically, used in asphalt surface recycling relates to the inability to convey sufficient amounts of the hot air to the asphalt surface to enable heat transfer to take place to the desired temperature and depth in the asphalt surface. The principal reason for this is the size and hot air throughput (e.g. cubic feet per minute or "cfm") necessary to expose the asphalt surface to sufficient heat for a sufficient period of time to heat the surface at a commercially viable rate of speed (e.g. 10-30 feet/minute) makes it impractical and/or prohibitively expensive to build a commercially useful apparatus. The result of this is that, in the asphalt surface recycling art, hot air and convection heaters are not commercially viable when compared to radiation heaters.
It would be desirable to have a method and apparatus for heating an asphalt surface which method and apparatus overcome or reduce at least one of the above-identified disadvantages of the prior art.